Direct methanol and H2/O2 proton exchange membrane (PEM) fuel cells are promising power generators for terrestrial and space applications where high energy efficiencies and high power densities are required. A critically important component of these devices is the proton conducting membrane. For a cationexchange membrane to be used in such fuel cells, a number of requirements are to be met, including: (I) High ionic (protonic) conductivity, (ii) dimensional stability (low/moderate swelling), (iii) low electro-osmotic water flow, (iv) mechanical strength and chemical stability over a wide temperature range, (vi) a high resistance to oxidation, reduction, and hydrolysis, and (vi) low hydrocarbon fuel cross-over rates (e.g., low methanol cross-over for direct methanol fuel cells). To date, those membranes reported in the open literature that conduct ions (protons) at moderate temperatures also possess a high methanol permeability and those membranes that do not transport methanol have a low proton conductivity.
Over the past decade, numerous membrane materials have been examined for use in hydrogen/oxygen and direct methanol fuel cells, including perfluorosulfonic acid membranes, such as Dupont""s Nafion(copyright) (see, for example, Ticianelli, Derouin, Redondo, and Srinivasan, 1988, J. Electrochem. Soc., 135, 2209), radiation-grafted copolymers of poly(styrene sulfonic acid) with either low-density poly(styrene), poly(tetrafluoroethylene)/poly (perfluoropropylene), or poly(tetrafluoroethylene) (Guzman-Garcia, Pintauro, Verbrugge, and Schneider, 1992, J. Appl. Electrochem., 22, 204), xcex3-radiation-grafted cation-exchange membranes where styrene/divinylbenzene was grafted into poly(fluoroethylene-co-hexafluoropropylene) (Bxc3xcchi, Gupta, Haas, and Scherer, 1995, Electrochim. Acta, 40, 345) and sulfonated styrene-ethylene/butylene-styrene triblock polymer (Wnek, Rider, Serpico, Einset, Ehrenberg, and Raboin, 1995, in Proton Conducting Membrane Fuel Cells I, S. Gottesfeld, G. Halpert, and A. Landgrebe, Eds., PV 95-23, The Electrochemical Society Proceedings Series, pp. 247-251). These polymers operate in a hydrated, water swollen state, which is necessary forfacile proton conduction. Unfortunately, the electro-osmotic water flows and methanol (liquid fuel) cross-over rates in these polymers are high. Additionally, some of the polymers are not chemically stable during long-time fuel cell operation (HO2xe2x80xa2radicals formed at the anode during oxygen reduction degrade the polymer).
Reinforced composite ion-exchange membranes have been used as proton-exchange materials in PEM fuel cells, where an ion-exchange polymer (normally a sulfonated perfluorinated polymer) is impregnated into a microporous polytetrafluoroethylene film (U.S. Pat. No. 5,525,436; Kolde, Bahar, Wilson, Zawodzinski, and Goftesfeld, 1995, xe2x80x9cProton Conducting Membrane Fuel Cells I,xe2x80x9d Electrochemical Society Proceedings, Vol. 95-23, p. 193). These composite membranes, which are identified by the GORE-SELECT trademark, are characterized by a high proton conductance and good mechanical properties, as is the case for homogeneous sulfonated perfluorinated polymer membranes. The methanol cross-over rates in homogeneous perfluorinated polymer membranes as well as the GORE-SELECT(trademark) membranes, however, are unacceptably high at methanol liquid feed concentrations greater than or equal to about 1.0 M.
Another material being examined as a fuel cell proton-exchange membrane is acid-doped polybenzimidazole (PBI) (U.S. Pat. No. 5,525,436). At elevated temperatures (greater than 100xc2x0 C.) these membranes exhibited good proton conductivity with low methanol cross-over rates. In contrast with traditional proton-exchange materials and the polyphosphazene membranes described in this patent application, the PBI membranes can not be used in a liquid feed methanol fuel cell because the acid dopant will leach out of the membrane and into the liquid methanol solution that is in contact with the membrane during fuel cell operation, resulting in a loss in proton conductivity.
Polyphosphazenes, whose basic structure is shown in FIG. 1, are an interesting class of polymers that combine the attributes of a low glass transition temperature polymer (a high degree of polymer chain flexibility) with high-temperature polymer stability. From a synthetic viewpoint polyphosphazenes are the most highly developed of all the inorganic-backbone polymer systems (see, for example, Potin, and DeJaeger, 1991 Eur. Polym. J., 27, 341). With appropriate functionalization of the phosphorous-nitrogen backbone, an unlimited number of specialty polymers can be synthesized. Thus, by the proper choice of R1 and R2 in the figure below, base polymers can be synthesized for eventual use in proton exchange membrane fuel cells (where the base polymer is chemically manipulated by the addition of sulfonate ion-exchange sites and/or chemical crosslinks). 
Polyphosphazenes (without fixed ion-exchange groups) have been used as pervaporation and gas separation membranes (see, for example, Peterson, Stone, McCaffrey, and Cummings, 1993, Sep. Sci. and Techn., 28,271) and as solvent-free solid polymer electrolyte membranes in lithium batteries, where there are no fixed charges attached to the polymer (Blonsky, Shriver, Austin, and Allcock, 1984, J. Am. Chem., Soc., 106, 6854). No one has yet used sulfonated polyphosphazene cation-exchange membranes as proton conductors in fuel cells (where water sorption is needed for trans-membrane proton transport).
From both theoretical predictions and experimental measurements, it is known that a proton-exchange membrane for solid polymer electrolyte (SPE) fuel cell applications requires a high concentration of ion-exchange groups and some water content for proton conduction. There are limitations, however, to the ion-exchange group concentration in the film, imposed by the required solvent transport properties of the membrane, the polymer chemistry, and the osmotic stability of the polymer. Thus, as the ion-exchange capacity of the polymer increases, water (and polar solvent) sorption by the polymer increases, resulting in unwanted polymer swelling (which may weaken the mechanical properties of the film) and unacceptably high liquid fuel (e.g., methanol) cross-over rates. It is also undesirable if the membrane water content were too low; a membrane""s ionic conductivity decreases dramatically when the average number of water molecules per ion-exchange site is less than six and a low polymer water content may also affect adversely the electrochemical kinetics of oxygen reduction during fuel cell operation.
Water and polar solvent (e.g., methanol) uptake in fuel cell proton-exchange membranes are difficult to control because many PEM materials are not crosslinked and the polymer""s water/methanol content is dependent on both the membrane""s ion-exchange capacity and the polymer crystallinity (which itself decreases with an increase in the number of fixed ion-exchange groups). Sulfonated polyphosphazene membranes (with SO3xe2x88x92 ion-exchange groups attached to the polymer) offer a much wider range of possible structures and water/methanol transport rates because the number of ion-exchange groups in the membrane can be adjusted independently of the degree of crosslinking. With a suitably sulfonated and crosslinked polyphosphazene membranes, the problems of unwanted water transport and methanol cross-over that are common to traditional PEM materials can be overcome, yet the membrane conductance can be maintained sufficiently high, since crosslinking limits swelling and water/methanol absorption and transport.
In addition to chemical crosslinking, there is another method by which the mechanical and transport properties of a polyphosphazene-based cation-exchange membrane can be altered and improved for SPE fuel cell applications, that being the blending of a sulfonated polyphosphazene with a non-sulfonated polymer. One can blend the sulfonated phosphazene with either a non-sulfonated polyphosphazene or some other polymer with good chemical and thermal stability, such as a high glass transition temperature (glassy) polyimide or polyetherimide. The non-sulfonated polymer in the blend swells minimally in water or methanol and thus provides a mechanically stable framework that constrains the swelling of the sulfonated phosphazene polymer component when the membrane is exposed to water and/or methanol. Low water and methanol transport will accompany the decrease in swelling of such physically crosslinked sulfonated phosphazene polymers. Additionally, the sulfonated and/or non-sulfonated components of the polymer blend may be chemically crosslinked in order to further adjust and enhance the mechanical and transport properties of the solid polymer electrolyte membrane.
Another technique to improve upon the mechanical properties of the polyphosphazene-based proton exchange membrane and to create very thin proton conducting films, is to impregnate a sulfonated polyphosphazene polymer or a polymer blend containing a sulfonated polyphosphazene into the void volume of a microporous support membrane. The polymeric material for the support membrane (e.g., microporous polyvinylidene fluoride) must be chemically and thermally inert at the operating conditions of a SPE fuel cell. The support membrane should also swell minimally when exposed to water and hydrocarbon fuel (e.g., methanol). After impregnation of a sulfonated phosphazene polymer solution into a microporous film and evaporation of solvent, the polyphosphazene can be crosslinked to further improve its structure and transport properties. Polyphosphazene crosslinking can be carried out, for example, by exposing a dry composite membrane to xcex3-radiation or by dissolving a UV-light photoinitiator into the polymer impregnation solution followed by exposure of the dry composite membrane to UV light.
The subject matter of this invention deals with sulfonated polyphosphazene-based cation-exchange membranes for PEM fuel cells where the polyphosphazene is crosslinked, noncrosslinked, suitably blended with one or more additional polymers, and impregnated into the void volume of an inert microporous membrane support and where the membranes operate in a hydrated state that is characterized by a high proton conductance and low water and methanol permeation rates.
Preliminary membrane fabrication experiments with selected phosphazene polymers have been reported in the literature. For example, solid-state UV radiation crosslinking of non-sulfonated ethylphenoxy/phenoxy substituted polyphosphazene films has been examined (Wycisk, Pintauro, Wang, and O""Connor, 1996, J. Appl. Polym. Sci., 59,1607). Also, non-crosslinked ion-exchange membranes were prepared from sulfonated methylphenoxy/phenoxy substituted phosphazene polymers (Wycisk and Pintauro, 1996, J. Membr. Sci., 119 155). In this latter study, it was shown that ion-exchange membranes could not be prepared from ethylphenoxy/phenoxy substituted phosphazene polymers, when SO3 was used as the sulfonating agent. In the above two studies, there was no specific attempt to fabricate a proton-exchange membrane from the sulfonated or crosslinked polyphosphazenes and the results provided no information as to the suitability of phosphazene polymers for fuel cell proton-exchange membranes. Individual membrane crosslinking and sulfonation experiments do not guarantee that one can either crosslink sulfonated polyphosphazene membrane, sulfonate a crosslinked membrane, or prepare a membrane by blending a sulfonated polyphosphazene and a non-sulfonated polymer. It is not possible to deduce from prior literature references, for example, whether a UV photo-initiator will solubilize in a dry phosphazene film when the polymer is partially sulfonated. Similarly, it is not known whether the presence of sulfonate fixed-charge groups on the polyphosphazene sidechains will interfere with the formation of UV-light-induced chemical crosslinks and whether the presence of polymer crosslinks will interfere with the sulfonation of the base polymer.
The subject matter of this invention relates to solid polymer electrolyte membranes comprised of a partially sulfonated polyphosphazene that conduct protons but exhibit a low methanol permeability when hydrated. The invention further relates to the use of such membranes in proton-exchange membrane fuel cells, such as hydrogen/oxygen and direct liquid-feed methanol fuel cells. In particular, the invention relates to polyphosphazene-based polymer electrolyte membranes that are comprised of one or more phosphazene polymers comprised of alkylphenoxy and/or phenoxy sidechains, where some portion of the these sidechains are sulfonated and where the sulfonated polymer is either non-crosslinked, crosslinked, blended with a non-sulfonated (or minimally sulfonated) polymer with no crosslinking, blended with a non-sulfonated (or minimally sulfonated) polymer with crosslinking, or impregnated into an inert microporous membrane support (with and without blending and/or phosphazene crosslinking).